1. Field of the Invention
This invention relates to a magnetic disk for magnetic recording in a direction parallel to or perpendicular to the surface thereof.
2. Background Art
One example of a conventional magnetic disk is a rigid magnetic disk. The substrate of the rigid magnetic disk is made of rigid material. In general, the rigid magnetic disk employs an aluminum substrate (JIS A5086 for instance).
The rigid magnetic disk is generally formed as follows. An aluminum plate is machined on a lathe, and its surface is polished so that a data recording operation is performed at high density with the space minimized between the disk and the head. Thereafter, a magnetic layer is formed thereon by vacuum deposition or spin coating. For high density recording and reproducing, the surface of the disk should be smooth. However, it has been difficult for the conventional magnetic disk having the aluminum substrate to provide a surface of not more than 0.1 micrometers in central line average roughness Ra. Furthermore, since the substrate is rigid, informing the magnetic layer, the continuous coating is limited by the web pass. That is, handling of the conventional rigid substrate is rather troublesome. On the other hand, a high density recording operation is greatly affected by the dust on the disk. Therefore, it is essential to manufacture magnetic disks in a dust-proof environment Accordingly, the magnetic disk manufacturing process is considerably intricate, thus requiring an enormous sum for investment in equipment.
The conventional substrate, such as an aluminum substrate, is rigid. Also, the head should not contact the magnetic layer in tracing the magnetic layer. However, it is difficult to maintain constant the narrow space between the head and the magnetic layer, and therefore signal errors occur frequently. And it is considerably difficult to further reduce the space between the head and the disk surface to thereby increase the recording density. If the magnetic head is accidentally brought into contact with the disk during the tracing operation with the narrow space maintained, a sliding frictional force is produced because the substrate is rigid. As a result, a great impact is applied collectively to the part of the magnetic layer which has contacted the head, which can break the disk surface, thus reducing the service life of the disk.
Furthermore, the aluminum substrate polished as described above is expensive.
On the other hand, a magnetic disk as shown in FIG. 1 has been proposed in the art. In the magnetic disk, a wide annular recess or groove 3 is formed on each side of a disk substrate 1. Floppy disks or flexible disks (hereinafter referred to as "flexible disk sheets") have a magnetic layer 5 on one side of a support 4. The support 4 with the attached magnetic layer 5 is bonded to an inner annular bonding ridge 6a and an outer annular bonding ridge 6b. An air gap is thus formed in the recess or groove 3 beneath the support 4. The magnetic layers 5 are exposed on the outside. Data is recorded on portions of the magnetic layers 5 over the grooves 3.
Heretofore, the substrate is made of metal such as aluminum or aluminum alloy, glass or polymer which were selected because they cause minimal thermal deformation.
Examples of the polymer are polycarbonate, polyphenylene sulfide, polysulfide, polyimide, polysulfone, polyacrylate, polyether sulfone, polyether imide and polyether ether ketone.
The magnetic recording surfaces of the conventional magnetic disk thus constructed are flexible. Therefore, even when the magnetic head is accidentally brought into contact with the magnetic recording surface, or when a higher density recording operation is carried out with the magnetic head held in contact with the magnetic recording layer, the magnetic layer is scarcely broken, unlike that of the rigid disk. Accordingly, the technique of flexible disks can be utilized as it is. Furthermore, a smooth magnetic layer which is high in durability can be used as a magnetic disk's magnetic layer. Thus, the magnetic disk is being watched with keen interest, eliminating the difficulties accompanying a conventional rigid magnetic disk.
The inventors have conducted intensive research on the magnetic disk of this type, and found that it still suffers form serious difficulties. That is, the air is the gaps is sealed when the flexible disk sheets are bonded to the substrate. Therefore, as the ambient temperature changes or the magnetic disk is continuously used the air in the gaps changes in volume (expanding or contracting), thus affecting the effective conditions of the magnetic head output. In other words, the effective tensile force of the flexible disk sheets is changed by the expansion or contraction of the air. As a result, the thickness of the air layer flowing through the space between the magnetic head and the flexible disk sheet is changed. That is, the spacing loss of the magnetic head is changed, thus affecting the magnetic head output.